Continuously variable transmissions (CVTs) are commonly used on a wide range of vehicles, such as small cars or trucks, snowmobiles, golf carts, scooters, etc. They often comprise a driving pulley mechanically connected to a motor, a driven pulley mechanically connected to wheels or a track, possibly through another mechanical device such as a gearbox, and a trapezoidal drivebelt transmitting torque between the driving pulley and the driven pulley. A CVT changes the ratio within certain limits as required by the operating conditions to yield a desired motor rotational speed for a given driven pulley rotational speed, the latter being generally proportional to the vehicle speed. A CVT may be used with all kinds of motors, for instance internal combustion engines, electric motors, etc. CVTs can also be used with other machines that are not vehicles.
Each pulley of a CVT comprises two members having opposite conical surfaces, which members are called sheaves. One sheave, sometimes called “fixed sheave”, can be rigidly connected to one end of a supporting shaft while the other sheave, sometimes called “movable sheave”, can be free to slide and/or rotate with reference to the fixed sheave by means of bushings or the like. The conical surfaces of the sheaves apply an axial force on the drivebelt. Moving the sheaves axially relative to each other changes the drivebelt operating diameter, thus the ratio of the CVT.
In order to transmit the motor torque, an axial force has to be applied in the driving and the driven pulleys. These axial forces can be generated by a plurality of possible mechanisms or arrangements. In a legacy mechanical CVT, the axial force in the driving pulley is often generated using centrifugal flyweights, spring and ramps.
Generally, at a low vehicle speed, the operating diameter of the drivebelt at the driving pulley is minimal and the operating diameter at the driven pulley is maximal. This is referred to as the minimum ratio or the minimum ratio condition since there is the minimum number of rotations or fraction of rotation of the driven pulley for each full rotation of the driving pulley.
As the vehicle speed increases, so does the driven pulley rotational speed. For a given operating condition, a certain motor rotational speed is desired, thus a desired ratio can be calculated. The CVT actuation mechanism is provided to set the CVT to the appropriate ratio.
At the maximum vehicle speed, the ratio is generally maximum as there is the maximum number of rotations or fraction of rotation of the driven pulley for each full rotation of the driving pulley.
Some driving pulleys are provided with an integrated clutching function. The clutch function can be provided directly on the drivebelt or be provided by a mechanism incorporated in the driving pulley. For instance, when the driving pulley has a clutching function on the drivebelt, the opposite walls of the sheaves can be designed to be sufficiently away from one another that they are not in a torque-transmitting engagement with the sides of the drivebelt. Then, when the operating conditions are such that clutching is required, the actuation mechanism of the driving pulley moves the sheave walls closer relative to each other. The sheave walls eventually make contact with the sides of the drivebelt. At this point, an axial force is applied by the actuation mechanism on the drivebelt. The amount of torque transferred to the drivebelt is somewhat related to this axial force applied by the actuation mechanism. At one point, enough friction is generated between the sheave walls and the drivebelt to produce a significant force transfer between the driveshaft and the drivebelt, thereby causing torque from the motor to be transferred as a driving force on the drivebelt. This driving force is transferred to the driven pulley of the CVT. Other models of driving pulleys can comprise a clutching function involving two or more contact surfaces.
The operation of the CVT as described above can be somewhat divided in three modes of operation. The first mode of operation is the unclutched mode of operation, where the driveshaft can rotate but no torque is transmitted to the drivebelt. The second mode is the fully clutched mode of operation, where there is a torque-transmitting engagement between the driveshaft and the drivebelt. The third mode of operation is the clutching (transitional) mode of operation when the driving pulley is between the two other modes.
The clutching mode of operation generally produces a slippage, for instance between two components such as the sides of the drivebelt and the sheave walls, or between the contact surfaces. It is often desirable to minimize such slippage, for instance to reduce wear and to improve the reaction time of the CVT. However, moving the components too quickly can result in undesirably sudden accelerations and noise, for instance. Mitigating the axial impact during the clutching mode is one of the challenges engineers face in the design of driving pulleys. Room for improvements always exists in this area.